1. Field of the Invention
The present invention concerns a method of and an apparatus for injection a molding light metal alloy such as of magnesium and aluminum.
2. Description of the Related Art
As a method of molding light metal alloy materials in a manner similar with injection molding of resins, there has been known a method of injecting a light metal alloy material in a semi-solidified slurry into a molding die. Generally, raw metal alloy materials are formed into a semi-solidified state by heating a raw material pellet in a screw extruder as disclosed in International Patent Publication Hei 3-504830 or by granulating raw ingot materials heated into a semi-molten state and then heating the same in a screw extruder as disclosed in Japanese Patent No. 2832625 or Japanese Laid-Open Hei 9-108805.
Since the starting materials are solid metals, any of the methods described above involves a problem that abrasion or flexion occurs violently in the upstream of the extrusion screw and a load torque has to be increased or a heating and stirring channel has to be enlarged in the screw extruder thereby making the size of the apparatus larger.
Further, since the solid material and the semi-solidified slurry are existed together in the axial direction of the extruder, metering upon extrusion tends to be instable. Further pores are liable to be mixed in molding products due to involvement of an inert gas to result in defective products.
In order to overcome the foregoing disadvantages, it has been proposed a method of cooling a molten metal in a vertical chamber under shearing by an extrusion screw into a semi-solidified slurry and then injecting the semi-solidified slurry discharged from a discharge port at the lower end of the chamber into a molding die (rheo-molding method: refer to International Patent Publication Hei 9-508850).
However, since the discharge port at the lower end of the vertical chamber is directly connected detachably to an upper portion of a molding die disposed therebelow, this method involves a drawback that the height of the entire apparatus is excessively large to increase the machine cost and also increases the maintenance cost.
In particular, in a case where the molding die is enlarged along with the enlargement of the size of molding products, it is necessary to locate a driving system such as a motor and a cylinder connected to the upper portion of the screw extruder and a storage hopper for the molten metal further higher, and such arrangement is extremely instable as a casting facility for actual operation.
Further, in the injection molding system described above, since the slurry is injected by rapidly lowering the extrusion screw in the material in which the liquid phase and the semi-solidification phase are mixed together, this involves an inherent problem that a screw flight is abraded violently and the slurry deposited in the upper portion of the screw tends to damage the shaft seal portion.
On the other hand, in the system separating an injection plunger described in Japanese Patent Laid-Open Hei 9-103859, since the nozzle at the top of the injection plunger is directed horizontally, when the nozzle is connected to the side of a molding die, the height for the entire apparatus can be reduced to some extent compared with the injection molding system described in the International Patent Publication Hei 9-508859.
However, in the system with a separated injection plunger described above, since a large melting furnace (feeder 20 with heater 25 shown in FIG. 1 of Japanese Laid-Open Hei 9-103859) is connected directly to an upper portion of the chamber, there is a limit for making the compact machine. Further, since a melting furnace for heating the solid material into a molten metal is directly connected to the upper portion of the chamber in this system, it is not favorable in view of the thermal stability of the molten metal and the safety. Further, when the melting furnace is connected to the upper portion of the chamber, it results in an inherent problem that the flow rate control is difficult